We have investigated the stable sites of interstitial oxide ions and the oxide-ion conduction mechanism in lanthanum germanate, La-10(GeO4)(6)O-3, by using first-principles calculations. Most stable positions of interstitial oxide ions are located at the center of two neighboring GeO4 tetrahedra. From first-principles molecular dynamics (FPMD) calculations, cooperative migration processes involving two or more oxide ions at interstitial and neighboring regular sites were observed, the so-called interstitialcy mechanism. This mechanism along the c axis was more frequent than that in the ab plane, which indicates the anisotropic oxide-ion conductivity in lanthanum germanate. (C) 2014 Elsevier B.V. All rights reserved.